新疆西南天山乌拉根铅锌矿床成矿机制研究：硫化物显微结构、元素与硫同位素组成制约

A large number of Zn-Pb deposits have been found in the sandstone of Mesozoic-Cenozoic continental redbeds, Southwestern Tianshan, Xinjiang. These deposits are different from classical SEDEX type, MVT and SST deposits in mineralization characteristics, and may represent a new mineralization type. However, their ore-forming mechanism remains controversial, and the unanswered questions regarding the source, generation method and location of massive H2S and enrichment mechanism of ore-forming metals are critical for better understanding of the ore-forming mechanism. The Uragen super-large Zn-Pb deposit in the region is characterized by the typical mineralization style and provides an excellent example to study the origin of H2S and ore-forming mechanism. The proposed project will focus on the sulfide minerals with various occurrences, stages, and generations of the Uragen deposit based on detailed field survey and microscopic petrography observation, and using a combined study of QEMSCAN, SEM+EDS, EPMA, in-situ trace element and sulfur isotope analysis by LA-(MC)-ICPMS, complete the study of the following three aspects: (1) the microstructure and mineral chemical composition of sulfides; (2) hydrothermal alteration and ore-forming fluid processes and (3) meta-analysis of origin of H2S and mechanism of ore precipitation, in order to better understand the ore-forming mechanism and critical ore-controlling factors in the Uragen large-scale Zn-Pb mineralization. This study may provide additional nutrients in the knowledge of Zn-Pb mineralization and serve for the continuous breakthrough of zinc-lead prospecting in Southwestern Tianshan.

新疆西南天山产于陆相红层砂岩中的铅锌矿床，以乌拉根超大型矿床为代表，成矿特征有别于经典MVT、SEDEX型和SST铅锌矿床，可能代表砂岩容矿铅锌矿床的新类型，但对其成矿机制认识分歧显著，而大规模铅锌硫化物成矿所需的巨量H2S从何而来、以何方式于何处生成、铅锌金属如何富集成矿等问题是理解成矿机制的关键。本项目拟针对乌拉根铅锌矿床H2S成因与铅锌富集机理认识不清关键问题，在详细野外地质调查和岩/矿相学观测基础上，以矿区不同产状、矿化阶段和世代的硫化物为对象，通过QEMSCAN、SEM+EDS、EPMA、LA-(MC)-ICPMS微区原位微量元素和硫同位素等研究手段，开展硫化物显微结构与矿物化学组成及其变化、热液蚀变与成矿流体演化、H2S成因与铅锌富集机理三方面研究，旨在揭示乌拉根铅锌成矿机制和关键控矿要素，以期丰富和发展砂岩容矿铅锌成矿理论，并为新疆西南天山铅锌找矿持续突破提供理论支撑。

新疆西南天山产于陆相红层砂岩中的铅锌矿床，以乌拉根超大型矿床为代表，成矿特征有别于经典MVT、SEDEX型和SST铅锌矿床，可能代表砂岩容矿铅锌矿床的新类型，但对其成矿机制认识分歧显著，而大规模铅锌硫化物成矿所需的巨量H2S从何而来、以何方式于何处生成、铅锌金属如何富集成矿等问题是理解成矿机制的关键。项目在详细的野外地质调查和室内岩/矿相学观测基础上，综合运用TIMA、SEM+EDS、EPMA、LA-(MC)-ICPMS微量元素和微区原位S-Pb同位素、C-O同位素等多种测试分析，开展了硫化物显微结构与矿物化学组成、热液蚀变与成矿流体演化和H2S成因与铅锌富集机理三方面研究。硫化物显微结构和微区地球化学研究表明铁硫化物形成多早于铅锌硫化物，先后经历了草莓状黄铁矿、卵状或不规则状胶状黄铁矿和较自形板状白铁矿，其中前两者具有极低δ34S值（＜-30‰），而后者低δ34S值（5‰~-10‰）；闪锌矿经历了两期三阶段，早期（Sph1和Sph2）棕黄色-浅黄色，无荧光，具有高Fe-Mn-Ga-Ge-As-Tl含量、低Cd-Cu-Ag含量和高δ34S值（＞15‰），而晚期（Sph3）无色-浅白色，有黄褐色或天蓝色荧光，具有低Fe-Mn-Ga-Ge-As-Tl含量、高Cd-Cu-Ag含量和低δ34S值（＜5‰）。基于此，成矿所需H2S早期主要源于残余孔隙水硫酸盐热化学还原（TSR），而晚期则与阿尔塔什组膏盐层溶解硫酸盐的细菌硫酸盐还原（BSR）有关。不同期次硫化物微区Pb同位素组成变化范围小，与可能矿源层/岩Pb同位素组成对比发现下白垩统克孜勒苏群第五岩性段可能为成矿金属主要来源。黄铁矿Re-Os同位素定年和方解石U-Pb定年结果表明乌拉根铅锌矿床经历了晚始新世、渐新世末-中新世和晚中新世三期成矿，可能分别与印度板块-Kohistan-Ladakh弧联合板块与欧亚大陆碰撞、主帕米尔断裂（MPT）及帕米尔前缘逆冲断裂（PFT）远程效应有关。红层“漂白”发生于铅锌成矿前，与区域油气充注期赤铁矿色素被H2S还原溶解有关，高⅀S含量的“漂白”砂岩地段为寻找乌拉根式铅锌矿床有利地区。该研究不仅丰富和发展了砂岩容矿铅锌成矿理论，还为新疆西南天山铅锌找矿持续突破提供理论支撑。